In chemical laboratory techniques, heating substances by microwaves has played an increasingly important role since the energy therein is delivered directly to the substance to be heated and does not necessarily need to be carried to it by means of heat conduction through vessel walls, as is the case with external heating. For many applications in which high temperatures and/or high pressures are to be achieved, pressure-tight vessels are used herein.
An example of such an application is the process of breaking down a substance in acid described in an overview article by H. M. Kingston and L. B. Jassie in Analytical Chemistry 58 (1986), pp. 2534-2541. This article explains the breakdown of a substance into the smallest element-specific molecules or ions with the aid of one or several acids. These can subsequently be determined qualitatively or quantitatively in an element analysis. For an acid breakdown, in general, high temperatures are required. To achieve these high temperatures, the acid is heated in a closed vessel whereby high pressures are developed.
A device for the acid breakdown of a substance in the microwave field for element analysis is described in the German Patent application (examined) 39 19 601. Use of strong acids as breakdown reagents limits the user to reaction vessels of teflon or quartz. In order to be able to close such a reaction apparatus so that it is pressure-tight, it is encased in an outer container comprising synthetic materials. The outer container is cylindrically shaped with a pot-like vessel and a cap detachably connected with one another. The reaction vessel, referred to as an inner container in the following, is also cylindrically shaped and has a front-face opening, a cover for it, and sealing means. In order to close the inner container in such a way that it is pressure tight, a pressing plunger elastically braced against the outer container presses onto the outside of the cover. Heating takes place in the microwave field of a microwave oven, wherein the entire device is exposed to the effect of the microwave field. For that reason, all parts of the device comprise a material which is transparent to microwaves.
The above-described device has the disadvantage that during heating of the substance disposed in the inner container, the synthetic outer container is heated due to heat transmission and loses its stability at high temperatures. This is also the case when an inner container comprising entirely or partially a synthetic material is used. Therefore high internal temperatures should only be used over short periods of time with long cooling periods following thereafter.
In the case of long-term heating, in contrast, the temperature in the inner container should not exceed 200.degree.-300.degree. C.
However, there are many materials which are difficult to break down, such as for example fats, oils, coal, coke, and synthetic materials, which require higher temperatures over longer periods of time for their breakdown. The above-described device is not optimally suited for a breakdown of these materials. The time required for a breakdown with acid in general decreases with increasing temperature; many materials whose breakdown below the above-stated temperature limit value takes a long time, could be broken down in significantly shorter time at higher temperatures.